The invention relates to a projection lens system for projecting in an image space an image from an image source present in an object space. The lens system comprises from the image end successively a first, positive lens group and at least a second, negative lens group whose surface facing the image end is concave.
The image source may be in the form of a cathode ray tube or a light source and an electronically controllable flat display device built up of a matrix of picture cells which, dependent on the control signal, reflect or absorb light and pass or absorb light, respectively. Instead of being absorbed, the light may also be dispersed.
A projection lens system of this type is known from U.S. Pat. No. 4,526,442 and is used for projecting a scene in one colour on a projection screen. By superimposition of three differently coloured scenes on the projection screen, a colour picture, for example a colour television picture can be obtained. In the known projection lens system a third lens group in the form of a positive lens is arranged between the first and second lens groups. The second lens group comprises a plano-concave lens element commonly referred to as field curvature correction lens or "field flattener" which ensures that the so-called Petzval curvature of the two other lens groups is compensated for. The second lens group supplies substantially the entire power of the lens system and is therefore referred to as the main group. The most important function of the first lens group is to correct aberrations.
In an image projection system such as a color television projection system, it is desirable that the maximum possible amount of light supplied by the image source reaches the projection screen. To this end the projection lens system is required to receive the maximum possible amount of light from the image source, which means that this system must have a large numerical aperture and that it passes this light with a minimum attenuation. A projection television system further requires the overall length of this system to be as small as possible so that this system, possibly in a folded state, can be accommodated in a cabinet having dimensions acceptable for consumer uses. This means that the focal length of the projection lens system is to be as short as possible. Furthermore this lens system must illuminate a projection screen of maximum possible dimensions as uniformly as possible in order that a maximum possible picture of good quality is presented to the viewers. To achieve this the projection lens system must have a large field angle at the image end, particularly for correctly displaying the edges of the picture. This field angle is understood to mean the angle between the optical axis of the projection lens system and the central axis of a light beam directed towards the edge of the projection screen, which beam passes through the projection lens system with a still acceptable vignetting. This field angle is proportional to the quotient ##EQU1## The product in the denominator is also referred to as the image projection distance. This distance determines the required optical path length for the projection system.
In the projection lens system described in U.S. Pat. No. 4,526,442 a mirror bending the optical axis of the lens system over an angle of approximately 90.degree. is arranged between the first lens group and the third lens group. While the length of the projection lens system is thereby reduced, the focal length is still relatively large. The first and third lens groups must be at a sufficiently large distance from each other for the mirror to be placed so that the projection lens system has a considerable vignetting. While vignetting can be reduced by enlarging the diameter of the first lens element, this is disadvantageous due to the required quantity of lens material and the weight of the lens element.